The Collaborative International Dictionary of English v.0.48:

En passant \En` pas`sant"\ [F.]
   In passing; in the course of any procedure; -- said specif.
   (Chess), of the taking of an adverse pawn which makes a first
   move of two squares by a pawn already so advanced as to
   threaten the first of these squares. The pawn which takes en
   passant is advanced to the threatened square.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

en passant
    adv 1: incidentally; in the course of doing something else; "he
           made this remark in passing" [syn: in passing, en
           passant]
    n 1: (chess) a chess pawn that is moved two squares can be
         captured by an opponent's pawn commanding the square that
         was passed